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I just got a great deal on a two year old lightly used SuperMax 19-38 drum sander. Small custom window manufacturer bought it and then never really used it. Sold it to me for $400. Now I can thickness my own tops, sides and backs. Very excited. Here’s the problem/question. What grit sandpaper do you recommend for thinning spruce tops and rosewood sides and back’s typically. 60? 80? I assure you don’t need to go too fine as you will be doing most of the fine sanding later.

yes 80 is good

I don't know how hard it is to change the paper on those machines, but on my wide belt sander, if I have to go more than a couple of mm, I alwas start with 36 or 40. Anything finer and you're just wasting time and sanding belts, or sanding rolls in your case. Might be too much hassle to change the paper on your machine though.-Bob

as bob points out this is a thickness sander not a finish sander. I have a belt sander that makes changing grits east. Drums not so much.
one thing that also affects the out come is feed rate. I agree that resinous woods like rosewood need coarse. Its Heat that is the enemy. heat pulls this out and it glazes on the paper. Many think finer is better but this is not the cases as it builds up a lot of heat.
If I am doing Rosewood 36 will take a lot off but leaves a coarse finish. so use that to get you close then 60 or 80 to finish off. one more time be aware of the heat. If your board is ware you need higher speed or coarser paper.
Early days I had a drum sander with 2 drums. It was a Grizzly I used to use a 40 on the in drum and 60 on the out.
Do take a lot when you remove. after a few passes feel the wood and the drum the should not be hot. play with feed and how much you take off. If you have an adjustable feed you can play with the speed and removal depth.

I use #80, I think I'd start there and see how it goes.

I have the same machine. I find (with a little practice) it's easy to change the paper. For tops, i lightly plane the surface with a hand plane if it's rough. Then I use 80 grit to level and thin a bit, and then to 120 grit to get close, but leave a few thou to work with. I use a ROS once the guitar is ready for finishing. For B&S I use 60 to start and then 80 and 120. Again, you have to leave a little for finishing. It's always good to tape some poster board to the show face of the top, so you don't damage it before assembly. Minor dents you can steam out, but a deep scratch can be a killer that you can't sand out. DAMHIK. Better to stay on the safe side. You really got a steal for $400.

36 grit, whowee that's some serious sanding. I've got that same sander (wish I'd paid $400) just putting that course of paper on would be a challenge. 80 grit works ok for me on most woods, but as John said some woods gum up real quick, just cost of using this method instead of some other way. Seem to find new uses for the machine weekly one of the best things I've ever bought, digital camera the #1.